The present invention relates a correction circuit for improving feed forward or feedback regulation in switched power supplies. In particular, the present invention relates to a correction circuit that measures a difference between inductor current and output current and provides a correction parameter to quickly adjust the duty factor of the pulse width modulation of the switched power supply to correct for this difference.
Control of output voltage in switching power supplies has long been an important consideration. For most applications, it is desirable to maintain output voltage and current at a more or less steady value, or within a desired window of values. For example, power supplies that are used as microprocessor power supplies must stay within a rather narrow window despite rather fast load transients on the output end thereof.
A common means for controlling output voltage is the use of feedback. However, these feedback control systems often require a tradeoff in stability in order to allow for improved transient response. This is due to the relatively fixed relationships between operating frequencies and control loop crossover frequencies.
Several approaches have been considered in order to address this shortcoming. One approach is to raise the supply operating frequency while lowering the value of the output inductor value. Another option is to add a “load line” which effectively raises the output impedance of the power supply to more effectively use the available window of output voltage values. Another idea is to add capacitors at the output in order to ensure that the output voltage remains in the desired window of values. Further, the use of output voltage excursions to trigger circuitry that rapidly changes duty factor has also been considered.
Another solution is contemplated by U.S. Pat. No. 6,753,723, the entire contents of which are hereby incorporated by reference herein. In this reference, the synchronous FET is turned off during a step down load transient in order to increase the voltage available to change the inductor current. While all of these approaches improve the situation, they add additional components, cost or complexity and typically do not provide a fast enough transient response. At best, they react to a problem (over/under voltage) that has already occurred.
U.S. Pat. No. 6,954,054, which is hereby incorporated by reference herein, contemplates a feed forward control approach that also attempts to address the problem of voltage control. While the feed forward control loop proposed in this reference allows for a quick response to the final duty factor, there is still a delay in the time that it takes for the inductive storage element(s) to reach a level appropriate for the new load current. The response time is related to the RL time constant of the power supply storage inductor and the equivalent resistance of the power supply. In other words, the droop in output voltage during load transient is what powers the change in the inductor current.
Thus, it would be beneficial to provide a control circuit for a switching power supply that avoids the problems discussed above.